1. Technical Field
The disclosure relates to a micro-electro-mechanical system (MEMS) device. More particularly, the disclosure relates to an MEMS device with oscillating assemblies.
2. Description of Related Art
In recent years, due to the popularity of electronic devices such as smart phones, tablet PCs, and game consoles, the market of MEMS inertia sensors such as accelerometers and gyroscopes (angular velocity meter) has greatly increased. Thus, international companies have invested significant resources to develop high performance and low cost MEMS inertia sensors. With the development of accelerometer technology, a new generation of the angular velocity meters with high performance and low cost has become the main competing product on the market for MEMS inertia sensor vendors.
FIG. 9 is a schematic structural view of a conventional MEMS gyroscope. The MEMS gyroscope 30 includes a proof mass 31, a driving electrode 32, a sensing electrode 33, a suspension beam 34, an anchor 35, and a substrate 36. The proof mass 31 is suspended above the substrate 36 by the suspension beam 34 and the anchor 35.
In a normal condition, the proof mass 31 is driven by the driving electrode 32 to oscillate at natural frequency along an x-axis, and this is referred to as a driving mode of the MEMS gyroscope 30. When the MEMS gyroscope 30 is applied with an external angular velocity Wz along the z-axis, the Coriolis acceleration is generated along the y-axis, thereby. The Coriolis acceleration further affects the proof mass 31 such that it vibrates along the y-axis (sensing direction), and this is referred to as a sensing mode of the MEMS gyroscope 30. The vibration is then measured by the sensing electrode to further calculate the external angular velocity. However, when an additional external acceleration is applied along the y-axis, the proof mass will subsequently have an additional y-axis displacement. This causes the sensing electrode in the sensing mode to generate an additional output signal, which will interfere with the output signal of the MEMS gyroscope 30. A tuning fork type gyroscope can solve the interference of the gyroscope caused by the additional acceleration. FIG. 10A to FIG. 10C are schematic views of an equivalent system of a tuning fork type gyroscope, which depict the tuning fork type gyroscope in the different mode. Referring to FIG. 10A and FIG. 10B, the tuning fork type gyroscope 40 includes two proof masses 41 and 42, a driving electrode 43, a sensing electrode 44, a support spring 45, and an anchor 46.
The proof masses 41, 42 are driven by the driving electrode 43 so that the two proof masses 41, 42 oscillate in opposite directions along the y-axis (e.g. a proof mass 41 moves in the positive y-direction, and the other proof mass 42 moves in the negative y-direction, as seen in FIG. 10A), and give rise to resonance simultaneously (this is referred to as a driving mode). When an external angular velocity Wz is applied along the z-axis, the Coriolis acceleration is induced on the x-axis. The Coriolis acceleration causes the proof masses 41, 42 to oscillate in the opposite directions along the x-axis (this is referred to as a sensing mode). The vibration is then measured by the sensing electrode 44 to further calculate the external angular velocity.
Referring to FIG. 10C, when an additional external acceleration Fa is applied to the tuning fork type gyroscope 40 along the y-axis, the two proof masses 41, 42 will both move along in the positive y-direction or in the negative y-direction. The changes of the capacitance of the tuning fork type gyroscope 40 are detected through differential calculations, so the variation of the capacitance caused by the additional acceleration is zero. Similarly, when the additional external acceleration is applied to the angular velocity meter along the x-axis, the two proof masses 41, 42 will both move in either the positive x-direction or the negative x-direction. This causes the variation of the capacitance to be zero. Thus, the tuning fork type gyroscope 40 can restrain the effect of an additional external acceleration Fa by its structure.
The technology of manufacturing tuning fork type angular velocity meters has two bottlenecks. First, it requires high-precision manufacturing process, and secondly the unexpected lateral displacements are prone to be happened due to variation of the manufacturing process. For example, a tuning fork type gyroscope 40 mainly includes two proof masses 41, 42, and a corresponding support spring 45. When the variations of these parts exist due to the manufacturing, the two proof masses 41, 42 or the spring constant of the support spring 45 do not match. At this point, the natural frequencies of the two proof masses 41, 42 are different, therefore the two proof masses 41, 42 can not resonate and oscillate in the opposite directions with respect to each other simultaneously (i.e. the phase difference is not 180 degrees). The effect of the additional external acceleration can not be eliminated by differential calculation. Consequently, the manufacturing variations described above may affect the sensitivity of the tuning fork type gyroscope 40, or even cause the tuning fork type gyroscope 40 to be malfunctioned.
FIG. 11A is a schematic diagram of a tuning fork type gyroscope of U.S. Pat. No. 7,043,985. FIG. 11B and FIG. 11C are schematic views depict respectively the motion of the equivalent systems of the tuning fork type gyroscope in the FIG. 11A. FIG. 11C only shows a portion of FIG. 11A. Referring to FIG. 11A to FIG. 11C, the tuning fork type gyroscope 50 includes two proof masses 51 and 52, a driving electrode 53, a sensing electrode 54, a support 55, a spring linkage 56, and an anchor 57. The difference between FIG. 10A to FIG. 10C is that the tuning fork type gyroscope 50 of FIG. 11A is connected to the anchor 57 through an end of the support 55. The other end of the support 55 is connected to a center region of the spring linkage 56, to reduce the amount of lateral displacement.